Proportioning girth reduction in high frequency welding of copper or copper base alloy tubing

ABSTRACT

In welding metal strip into tubing, the strip is formed into an open tube whose edges define a longitudinally extending gap and are brought together at a weld point. The girth of the tube at the weld point is reduced to substantially squeeze out the molten metal and form a force weld. The degree of girth reduction is proportioned to the ratio of tube wall thickness to tube outside diameter such that the girth reduction falls within the region delineated by the area ABCD, and preferably by the area CDEF in FIG. 3, for ratios up to 0.08. For ratios in excess of 0.08, the girth reduction is at least 0.080 inch and, preferably, from 0.080 to 0.090 inch.

United States Patent Yungbluth 3,701,876 [451 Oct. 31, 1972 [54] PROPORTIONING GIRTH REDUCTION IN HIGH FREQUENCY WELDING OF COPPER OR COPPER BASE ALLOY I IN Primary Examiner-J. V. Truhe Assistant Examiner-L. A. Schutzman [72] Inventor: aussell L. Yunghluth, Florrisant, Ammey Robe1-t Bachman et aL o. [73] Assignee: Olin Corporation [57] ABSTRACT 22 Filed; June 21, 1971 v In welding metal strip into tubing, the strip is formed I into an open tube whose edges define a longitudinally [21] PP N05 154,753 extending gap and are brought together at a weld point. The girth of the tube at the weld point is 52 67 29 47 reduced to substantially squeeze out the molten metal 1 19/ I 7 and form a force weld. The degree of girth reduction [51] u CL 323k 31/06 is proportioned to the ratio of tube wall thickness to [58] Field tube outside diameter such that the girth reduction k 2 falls within the region delineated by the area ABCD, 7 and preferably by the area CDEF in FIG. 3, for ratios up to 0.08. For ratios in excess of 0.08, the girth [56] References Cited reduction is at least 0.080 inch and, preferably, from UNITED STATES PATENTS 008010 (1090 3,015,018 12/1961 Rudd ..219/67 I 2 Claims, 3 Drawing Figures vs B ."LC F 3 Gas D u 5 M E JIJ ,040

TUBE WALL THICKNESS TO TUBE OUT SIDE 014m ER RATIO PATENTEDUCT 31 m2 3.701. 876 sum 10F '2 INVENTOR I RUSSE LL L. VUNGBLU TH MA/M ATTORN EY G/RTl-I REDUCTION INCHES PATENTEDUCI 3 1 I972 I saw a or 2 .IOO .090 I F M .oao C 4Q .070

FIG-'3 RUSSELL LYUNGBLUTH I INVENTIOR ATTORNEY PROPORTIONING GIRTH REDUCTION IN HIGH FREQUENCY WELDING OF COPPER OR COPPER BASE ALLOY TUBING BACKGROUND OF THE INVENTION In the art of welding metal strip into tubing, it is known to use high frequency induction welding. A typical example of a prior art apparatus and process is set out in US. Pat. No. 3,037,105, granted May 29, I962. The welding process disclosed therein employs a forge welding technique wherein thereis a reduction in the girth of the tube at the weld rolls.

In accordance with the known practices, the reduc- I SUMMARY OF THE INVENTION In accordance with'this invention, it has been found for copper and copper base alloys that the amount of squeeze out or girth reduction has a significant effect on weld quality. Further, it has been. found that the amount of squeeze out must be limited within specific ranges for given tube wall thicknesses to tube outside diameter ratios. 3

Therefore, this invention comprises proportioning the squeeze out at the weld rolls to the thickness to diameter ratio such that the degree of squeeze out employed falls within the area ABCD of FIG. 3 and, preferably, within the area CDEF of FIG. 3.

It is accordingly an object of this invention to provide a process for welding copper or copper base alloy strip into tubing wherein the girth reduction at the weld rolls is proportioned to the ratio of tube wall thickness to tube outside diameter of the tube being welded.

It is a' further object of this invention to provide a process as above wherein the degree of girth reduction at the weld rollsversus the thickness to diameter ratio falls within the range ABCD of FIG. 3 and, preferably, within the range CDEF. I y 7 Other objects and advantages will become apparent from the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of a typical prior art high DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and especially to FIG. 1, there is illustrated a typical welding apparatus 1 for welding metal strip in the form of a preformed. open tube 2 into tubing. The apparatus is a adapted to weld together the opposed edges 3 and 4 which form the longitudinal gap 5 in the open metal tubing 2.

The open tube 2 is formed in a forming mill (not shown) situated in line immediately preceding the apparatus l. The forming mill is of conventional well known design. The forming mill may contain a plurality of roll stands or dies which form metal strip into the open tube 2. It is to be understood that other roll stands may be situated in line immediately following the apparatus l to further shape or size the tubing such as, for example, to correct out of roundness. The roll stands are generally power driven and, therefore, provide means for rapidly and longitudinally advancing the tubing. Alternatively, this means may be power driven weld rolls 6 as shown.

The weld rolls 6 comprise a pair of pressure rollers of known form which engage opposite sides of. the tube 2 and cause the longitudinally extending gap 5 therein to become closed substantially at a weld point 7 forming a 'V-shaped gap 8. As. the open tubing 2 advances to the weld point 7, the edges 3 and 4 at the gap 8 become welded together along the weld seam 9.

The edges'3 and 4 at the V-shaped gap 8 are heated by. means of an induction coil 10 or contacts. The induction coil 10, shown is a single turn coil; however, a

.multiturncoil or other appropriate device could be substituted for the single turn coil.

The coil 10 is formed of wrought metaltubing. The coil is electrically connected to a source of high frequency current, not shown. The high frequency current is normally at least 10 kilocycles per second and, preferably, at least kilocycles per second or higher. The coil is also connected to a source of cooling medium (not shown) which flows through the tubing to keep it from overheating.

The-apparatus 1 may also include the following elements as shown in FIG. 1. An impeder I may be included to improvethe efficiency of the induction coil 10 by increasing the impedence of the high frequency current paths around the back of the open tube 2. This reduces the flow of current around the back of the tube 2 and increases the flow of current along the tube edges Sand 4 running from a point on the tube edge 3 adjacent the coil 10 to the weld point 7 and back the opposing tube edge 4 to a'point adjacent the coil 10. This results in more efficient heating of the edges 3 and 4 of the open tube'2, the edgesbeing heated up to the welding temperature upon reaching the weld point 7.

The impeder I may be of any suitably known connetic'material in the core should be of an insulating nature to provide a core substantially free of eddy current losses. The core is preferably a sintered magnetic oxide insulating material, preferably of types now well known which have a low loss factor and high volume resistivity. A suitable material is marketed under the name Ferrarnic by General Ceramic and Steatite Corporation, the permeability thereof being substantially greater than unity. The cooling medium which passes through the hollow supporting arm 12 flows within the schroud 11 and about the Ferramic core M to cool the core and is generally discharged within the welded tube 13.

Further, since the angle of the V-shaped gap 8 is important, a seam guide means 14 is generally included at a point in advance of the induction coil to accurately space apart the edges of the tube 2 and, thereby, obtain the desired angle. The seam guide means may be formed of a suitable insulating material protruding down into the gap so that the roll stand preceding that position (not shown) will cause the tube gap edges 3 and 4 to be pressed against the opposite sides of the seam guide 14 whereby the edges are maintained with a uniform separation.

The welding apparatus just described is but one type of apparatus useful with the instant invention and numerous other high frequency welding apparatus are known which could be employed in place thereof.

It has heretofore been thought that large amounts of squeeze out or girth reduction at the weld rolls 6 were not required to obtain sound welds. While this may be the case when welding metals having relatively high resistivities with respect to copper and having comparatively large temperature differentials between their solidus and liquidus points as compared to copper base alloys, it has been found in accordance with this invention that following prior art practices results in a number of difficulties when welding copper and copper base alloys.

When the degree of squeeze out is slight, say on the order of 0.030 inch, the amount of upset of the tube edges 3 and 4 at the weld rolls 6 is very small. Therefore, virtually the whole thickness of the tube edge as upset is heated up to welding temperature to obtain a weld over the full thickness of the tube wall. This results in a pool of molten metal at the weld point 7 which has the effect of moving the position of the weld point closer to the coil 10, thereby increasing the heat input to the tube edges 3 and 4 at theweld point 7 and other associated effects.

The changes in the heat input to the tube edges 3 and 4 require consequent changes in the power applied to the induction coil 10, thereby resulting in an inherent instability in the welding process and in nonuniform welds. The instability in the welding process can also result in surface portions of the weld seam 9 having a cast structure rather than a forged welded structure. Such a cast structure is undesirable and is susceptable to cracks, voids and poor mechanical properties. Further, the molten metal pool may under the influence of the magnetic field be at least partially blown out of the weld area resulting in a void which generally extends well into the tube wall.

To substantially eliminate the problem of voids and other defects encountered with the prior art process and to develop a more stable process which will yield uniform welds of high quality, in accordance with this invention the amount of girth reduction at the weld rolls 6 has been substantially increased. The result of increasing the girth reduction is shown schematically in FIG. 2. The weld structure shown therein is meant to be illustrative but not limitive of the instant invention and is not to scale.

As shown therein, the amount of upset of the tube edges 3 and 4 is substantial and considerably larger than would be obtained by the prior art process. It is therefore possible in accordance with this invention to control the power input to the coil 10 such that the whole thickness of the tube wall edges 3' and 4' as upset are not heated to welding temperatures. The thickness between the dashed lines 21 in FIG. 2 is the true wall thickness A which will be obtained after the upset portions 20 and weld beads 22 have been scarfed from the inside and outside tube surface 13 and 13.

In the induction heating process of this invention, the heat is most concentrated toward the middle 23 of the tube wall edges 3 and 4. Therefore, it is possible to adjust the power input to the coil 10 such that only a portion 24 slightly in excess of the true tube wall thickness A is heated to welding temperatures and thereby subjected to forge welding. The remaining portions 25 of the upset tube wall edges 3' and 4' are either unwelded or contain squeezed out molten metal from the forge welding zone 24 which at best forms a cast type weld.

Cracks or voids in the cast portions 25 of the weld appear in the upset portion 20 of the strip edges '3' and 4' which is scarfed away, thereby leaving a uniform void freeweld in the tube afterscarfing. Further, since the region of forge welding 24 is slightly greater than the true thickness A of the tube wall, slight instabilities in the heat supplied to the tube edges will not reduce the region'of the forge-welding sufficiently to have any substantial amount of cast material present in the tube wall after scarfing, thereby eliminating difficulties attendant to the presence of a cast weld portion 25 near the surface of the weld seam.

In view of the fact that the whole upset tube edge surface 20 is not heated to welding temperatures, no pool of molten'metal forms at the weld point 7 as with the prior art process, save for the metal which is squeezed out from the forge welding zone 24. Even if this squeezed out metal is blown out under the influence of a magnetic field, the resulting void would occur in the upset portion 20 of the tube wall and would, therefore, be scarfed off leaving a defect free weld.

The problem of instability of the welding process is further overcome because the absence of the floating molten metal pool as in the prior art process allows the weld point 7 to be maintained at a position substantially constant with respect to the induction coil 10.

A further advantage in accordance with the instant invention is the fact that the greater degree of upset provides a continuous strip of scarfing material which is easily disposed of both within and without the tube. The scarf is not susceptable to build up. In accordance with the prior art process, there may be insufficient upset area and weld metal bead to form a continuous strip of scarf'mg and instead individual particles are removed. These individual particles tend to build up and interfere with the welding process sometimes even requiring shutdown of the mill to clean out the resultant clogs.

To obtain the welds as shown in FIG. 2 in accordance with this invention it is essential that the squeeze out or girth reduction at the weld rolls be proportioned to the tube wall thickness to tube outside diameter ratio such that the girth reduction is maintained within the area ABCD of FIG. 4 and, preferably, within the area CDEF.

The areas delineated in FIG.3 are based on practical operating ranges of 0.015 to 0.095 tube wall thickness and k to 3% inches inclusive outside diameters. It should be observed that the preferred operating ranges flatten out between 0.080 inch and 0.090 inch squeeze out and this range of girth reduction is applicable to tube wall thickness to tube outside diameter ratios in excess of 0.08.

Therefore, in accordance with this invention when welding copper or copper base alloy tubing having a tube wall thickness to tube outside diameter ratio of 0.01 to 0.08, the ranges of girth reduction should be maintained within the levels set out in FIG. 3. For ratios in excess of 0.08, the range of girthreduction should be limited to at least 0.080 inch and, preferably, 0.080 to 0.090 inch.

It is to be understood that the invention is not limited to the illustrations described and shown herein, which are deemed to be merely illustrative of the best modes of carrying out the invention, and which are suitable of modification of form, size, arrangement of parts and details of operation. The invention rather is intended to encompass all such modifications which are within its spirit and scope as defined by the claims.

What is claimed is: 1. A process of welding metal strip into tubing comprising the steps of:

forming said strip into an open tube whose edges define a longitudinally extending gap;

bringing said edges together at a weld point; and

reducing the girth of said tube at said weld point to substantially squeeze out any molten metal from between said strip edges and, thereby, form a forge weld;

the improvement wherein: said metal is copper or a copper base alloy; and wherein in said step of reducing the girth of said tube, the girth reduction is proportioned to the ratio of tube wall thickness to tube outside diameter such that the girth reduction falls within the range delineated by the area ABCD in FIG. 3, for said ratios up to 0.08 and is at least about 0.080 inch for said ratios in excess of 0.08.

2. A process as in claim 1 wherein said girth reduction is maintained within the area CDEF for said ratios up to 0.08 and is maintained between about 0.080 inch and 0.090 inch for said ratios in excess of 0.08. 

2. A process as in claim 1 wherein said girth reduction is maintained within the area CDEF for said ratios up to 0.08 and is maintained between about 0.080 inch and 0.090 inch for said ratios in excess of 0.08. 